Semiconductor nanocrystals (NC) films manifest size-dependent tunable properties and can be deposited on a variety of substrates by facile wet-chemical methods. Therefore these materials have a high potential to serve as prospective materials in diverse application fields including transistor devices, functional circuits, sensors, photovoltaics and light emitting devices.
These applications require tuning of the electrical properties of the NC film to achieve efficient charge transport, along with patterning capabilities to construct functional circuits. Commonly, in bulk semiconductors lithographically patterned doping is ubiquitously used to fabricate complex microelectronic circuitry. In the case of semiconductor NC films, such control is still lacking. There is still a gap in methods of effective aliovalent doping of NCs.
Standard semiconductor doping methods have proven problematic to use with nanocrystals since the introduction of impurity atoms into NCs containing a few thousands atoms may lead to their expulsion to the surface via a “self purification” mechanism. Nevertheless, due to the central role doping can play in controlling the electronic properties, ongoing efforts have been reported for doping of semiconductor NCs.
For example, a first approach was to introduce excess charge carrier into the NCs via chemically altering their surfaces, realizing so called “remote doping”. This is achievable through the use of suitable electron donating ligands binding on the nanoparticle surfaces or through electrochemical carrier injection where both may generate semiconductor NCs with n-type doping characteristics.
Such a method, as well as many others, fall under either interstitial or substitutional impurities and require tailoring the electronic properties of an impurity material to that of the NC.
Defect formation in thin films of chalcogenides and chalcopyrites have been studied with estimations of the vacancy formation energies for bulk materials [1], but may present quite different values in their NC form.
Luther, J. M. et al. reported that an oxidation by oxygen exposure in ambient conditions of Cu2S NCs led to observation of a plasmon band assigned to free carries induced by copper vacancies [2,3].
Copper vacancies in various Cu-chalcogenides were shown to exhibit tunable optical properties, such as plasmons [2,3]. In addition to spherical NCs, different geometries of metal chalcogenides may influence vacancy formation criteria and the resulting properties. For example Cu2S nanorods have shown to present different onset temperatures for crystallographic transitions, with variations in the nano-rod's width [4]. The electrical properties of Cu2S nanorods have not been studied yet. However, the formation of vacancies and crystallographic transitions were observed [5]. Along these lines additional Cu2S shapes (tetrapods etc.) might also respond differently to the thermal doping process.
These low cost materials in their thin film morphologies were investigated intensively as prospects materials for photovoltaic applications, due to their band gap values (1.1-1.3 eV) and lack of toxic Cd content.
The conductance mechanism in NC arrays is strongly dependent on tunneling between adjacent NC through the insulating ligands the serve as a tunneling barrier. The tunneling process is exponentially sensitive to the distance between the NCs. Therefore, any reduction in this distance will also increase dramatically the conductance of the array. This can be achieved via annealing and removal of the organic protecting layer. This process referred to as annealing, is conducted at typical temperatures of 200-300° C. and is the standard method for enhancing conductance of NC devices by a few orders of magnitude (103-106) [6-13]. Although annealing is conducted at high temperatures most works ignore any induced changes to the NC's stoichiometry and defect formation and concentrate on the ligands layer by studying the changes in distance between NCs with small angle x-ray scattering (SAXS) and typical features of the ligand layer absorbance with optical spectroscopy.